In so many ways humans behave predictably. If you respond to a stimulus often enough, the response becomes automatic and the relationship very powerful. We’ve all heard of Pavlov’s famous dog which exemplifies the principle (actually, it goes a bit further). Human factors design takes this stimulus/reaction, along with many other factors, into account when creating warnings, switches, procedures and so forth.
Amazingly, there is one critical warning system in the Boeing 737 that is doomed to be mis-interpreted. It completely ignores human factors. It is the cabin altitude warning system.
Boeing 737 Pressurization panel and throttle quadrant
As a crew prepares for every single flight, they perform a check of the takeoff warning horn—an important warning intended to prevent attempting takeoff without all the controls properly set. The pilots move the throttles forward and listen for it’s distinctive “buzz…buzz…buzz.” Every flight—just before leaving the gate, push a throttle up and listen for that buzzing. So every time hear they “buzz…buzz…buzz” it’s related to the takeoff configuration warning. Every time. While taxiing out, they move the throttle lever up to make sure they do not hear that distinctive warning while glancing at the flaps to insure they’re set.
When cruising at altitude, losing pressurization can render a crew unconscious in seconds. While a rapid depressurization would be obvious from popping ears, hissing noises and probably fogging, a slow loss might be missed. Guess what the first indication the pilots will likely get of low cabin pressure? Yup, the “buzz…buzz…buzz” of a takeoff warning horn. Boeing uses the exact same sound to warn of a cabin pressure loss.
You’d like to think that trained pilots would know the difference. You’d be wrong. Remember, the pilots connect this noise on every single flight with the throttles/takeoff warning that they test before pushing back and while taxiing out.
It turns out, numerous pilots have gotten the warning during climb and did not realize what it was for. Upon hearing the “buzz…buzz…buzz” they immediately suspected an issue with the takeoff warning. In fact, they didn’t realize the pressurization was failing until the oxygen masks dropped (which occurs after the cabin has depressurized further).
Given the dire consequences of an unconscious crew, this situation is truly unfathomable. This error has been happening on 737’s since before I was flying them and is truly tragic that the system has not been modified. An operational change could help by acknowledging that the horn being tested is for both cabin altitude and takeoff warning. But the real fix is —a unique warning, preferably a voice that clearly states “cabin altitude”. This warning is too important to save money by using the same buzzer.
It appears that this same exact failing caused the loss of aircraft and occupants in the Greek 737 accident where they crashed with an unconscious flight crew.
Solution
After the Helios crash in 2005 Boeing and the airlines implemented measures to make sure it doesn’t happen again although it they weren’t fully implemented until about 2008 or later in all the fleet.
The solution was a red warning light that clearly specifies which problem the horn is sounding for. It must cost an inordinate amount of money to change the sound given how much better that would be (a voice that says “CABIN ALTITUDE HIGH.” The proof would be in how many times airline crews actually allow the cabin to climb so high the masks drop. I haven’t seen that data but haven’t heard about it, either, so I suspect that overall it’s now pretty rare.